This invention relates to sensors for detecting an analyte by luminescence quenching, and more particularly to oxygen-quenchable luminescent lanthanide complexes for an oxygen sensor, a combination measuring and reference analyte sensor containing both quenchable and nonquenchable luminescent materials, and a low-noise phosphorescent analyte sensor.
A blood gas analysis is performed on many hospital patients both during and after surgery. The three parameters of interest are the partial pressures of oxygen (PO.sub.2) and carbon dioxide PCO.sub.2), and the negative logarithm of hydrogen ion activity, the pH. These three parameters give a good indication of a patient's cardiac, respiratory and circulatory functioning, and the rate of metabolism. Monitoring the level of oxygen gas in the blood is important for determining the amount of oxygen being delivered to the tissues.
Several sophisticated blood gas analyzers are commercially available for analyzing blood samples after the blood is extracted from the patient (in vitro). However, the withdrawal and subsequent analysis of a blood sample is both cumbersome and time-consuming and does not permit continuous monitoring of the dissolved gases in a patient's blood. There has been a need for many years for a system which would enable blood gas measurements to be made directly in the patient (in vivo), thereby avoiding the difficulties and expense inherent in the in vitro techniques.
Among the suggestions in the prior art was the use of indwelling electrode probes for continuous monitoring of the blood gas. The in vivo electrode probes have not been generally acceptable. Two principal disadvantages of electrode probes are the danger of using electrical currents in the body and the difficulty of properly calibrating the electrodes.
Also among the suggested techniques for in vivo measurement has been the use of fiber optic systems. In a fiber optic system, light from a suitable source travels along an optically conducting fiber to its distal end where it undergoes some change caused by interaction with a component of the medium in which the probe is inserted or interaction with a material contained in the probe tip which is sensitive to (i.e., modified by) a component in the medium. The modified light returns along the same or another fiber to a light-measuring instrument which interprets the return light signal.
Fiber optic sensors appear to offer several potential benefits. A fiber optic sensor is safe, involving no electrical currents in the body. Optical fibers are very small and flexible, allowing placement in the very small blood vessels of the heart. The materials used, i.e., plastic, metal, and glass, are suitable for long-term implantation. With fiber optic sensing, existing optical measurement techniques could be adapted to provide a highly localized measurement. Light intensity measurements could be processed for direct readout by standard analogue and digital circuitry or a microprocessor. However, although the potential benefits of an indwelling fiber optic sensor have long been recognized, they have not yet been realized in widely accepted commercial products. Among the principal difficulties has been in the development of a sensor in a sufficiently small size which is capable of relatively simple and economical manufacture so that it may be disposable.
An oxygen sensor based on oxygen-quenched fluorescence is described in U.S. Reissue No. 31,879 to Lubbers et al. Lubbers et al. describe an optode consisting of a light-transmissive upper layer coupled to a light source, an oxygen-permeable lower diffusion membrane in contact with an oxygen-containing fluid, and a middle layer of an oxygen-quenchable fluorescent indicating substance, such as pyrenebutyric acid. When illuminated by a source light beam of a predetermined wavelength, the indicating substance emits a fluorescent beam of a wavelength different from the source beam and whose intensity is inversely proportional to the concentration of oxygen present. The resultant beam emanating from the optode, which includes both a portion of the source beam reflected from the optode and the fluorescent beam emitted by the indicating substance, is discriminated by means of a filter so that only the fluorescent beam is sent to the detector. In a second embodiment, the optode consists of a supporting foil made of a gas-diffusable material such as silicone in which the fluorescent indicating substance is randomly mixed, preferably in a polymerization type reaction, so that the indicating substance will not be washed away by the flow of blood over the optode. Lubbers et al. assert that both optodes can be adapted for in vivo use by disposing the same at the distal end of a catheter containing a pair of optical fibers for the incident and outgoing beams. However, the multi-layer optode of the first embodiment would be difficult to miniaturize. Lubbers et al. fails to disclose any method for attaching the alternative supporting foil to the distal end of the optical fibers or catheter. Furthermore, these sensors require at least two optical fibers which further limits miniaturization of the device.
Another PO.sub.l 2 sensor probe utilizing an oxygen-sensitive fluorescent intermediate reagent is described in U.S. Pat. No. 4,476,870 to Peterson et al. The Peterson et al. probe includes two optical fibers ending in a jacket of porous polymer tubing. The tubing is packed with a fluorescent light-excitable dye adsorbed on a particulate polymeric support. The polymeric adsorbent is said to avoid the problem of humidity sensitivity found with inorganic adsorbents such as silica gel. The probe is calibrated by using a blue light illuminating signal and measuring both the intensity of the emitted fluorescent green signal and the intensity of the scattered blue illuminating signal. Again, it is difficult to miniaturize the Peterson et al. sensor tip wherein a porous particulate polymer is packed within an outer tubing.
U.S. Pat. No. 3,612,866 to Stevens describes another method of calibrating an oxygen-quenchable luminescent sensor. The Stevens device, designed for use outside the body, includes an oxygen-sensitive luminescent sensor made of pyrene and, disposed adjacent thereto, an oxygen-insensitive reference sensor also made of pyrene but which is covered with an oxygen-impermeable layer. The oxygen concentration is evaluated by comparing the outputs of the measuring and reference sensors.
A principal disadvantage of the prior art sensors is their large size which prohibits their use in the narrow blood vessels, such as the narrow vessels of the heart or those of neonates.
Another disadvantage with the prior art oxygen sensors is that the detected luminescence signal includes a great deal of background noise in addition to the oxygen-quenched luminescence. The noise consists of reflections of the incident signal and broadband luminescence generated by other components in the system, such as the optical fiber. It would be desirable to eliminate the background noise in order to obtain a more precise measurement of oxygen concentration.
It is known that lanthanide ions can be excited to luminescent levels through energy transfer from excited ligands complexing the ions. The ligand absorbs energy to reach an excited singlet state and then may undergo a radiationless transition to an excited triplet state. A transfer of energy from the ligand to the lanthanide ion can occur if the energy of the singlet or triplet state exceeds that of the luminescent state of the lanthanide ion.
Because of their narrow-line emissions, luminescent lanthanide complexes have found widespread use as laser materials, surface coatings, and as identifying probes in protein analysis. See U.S. Pat. Nos. 3,484,380 to Kleinerman, 3,440,173 to Hovey et al., and 4,037,172 to Filipescu et al. The lanthanide complexes have not been considered for use as oxygen sensors because they are not considered oxygen quenchable. In A. Heller et al. "Intermolecular Energy Transfer From Excited Organic Compounds To Rare Earth Ions In Dilute Solutions," 42 J. Chem. Physics 949-953 (Feb. 1965), it was found that noncomplexed aromatic aldehydes and ketones in solutions with lanthanide ions exhibited oxygen quenching. The authors state that quenching by oxygen was not readily observed with chelates of terbium and europium. While certain proteins and amino acid ligands useful in biological systems, such as 1-(p-methoxybenzyl)-EDTA, have exhibited some oxygen quenching of lanthanide luminescence in aqueous solution, the quenching was small, even in the presence of an oxygen saturated solution. Thus, it was not apparent that such compounds were sufficiently sensitive even in an aqueous solution to produce a useable oxygen sensor, nor that such compounds immobilized in a solid matrix would exhibit any sensitivity at all. See F. Prendergast et al., "Oxygen Quenching Of Sensitized Terbium Luminescence In Complexes Of Terbium With Small Organic Ligands And Proteins," 258 J. Bio. Chem. 4075-4078 (1983); A. Abusaleh et al., "Excitation And De-Excitation Processes In Lanthanide Chelates Bearing Aromatic Sidechains," 39 Photochemistry and Photobiology 763-769 (1984).
It is a surprising aspect of this invention that certain lanthanide complexes have been found to be sufficiently oxygen quenchable for use as an oxygen sensor.
It is an object of this invention to provide oxygen-quenchable luminescent materials having very narrow emission bands for use as oxygen sensors.
Another object is to provide luminescent materials which are oxygen quenchable in solid form for use as oxygen sensors.
A further object is to provide methods of improving the oxygen sensitivy of the solid luminescent materials.
A further object is to provide oxygen-quenchable luminescent materials which can be excited at wavelengths above 300 nm and thus can be used with commercially-available optical fibers for constructing an in vivo oxygen probe.
Another object is to provide a combination measuring and reference analyte sensor containing both quenchable and nonquenchable luminescent materials.
A further object is to provide luminescent materials having a relatively long-lived luminescence or phosphorescence.
A still further object is to provide a low-noise oxygen sensor based on such phosphorescent materials.
Still another object is to provide a small diameter catheter containing a precalibrated, low-noise oxygen sensor for use in the narrow blood vessels of the body.